The present invention relates to an image forming an method for forming image on a light-sensitive material by use of a light-emitting diode array as a point-light source. More particularly, the invention relates to an image forming method for forming half-tone images on a light-sensitive material by gradation-controlling a light-emitting diode array.
Heretofore, a half-tone image (latent image) has been formed on a light-sensitive material, such as silver-salt film or the like, by exposing the light-sensitive material to light emitted from a light-emitting diode array composed of tiny light-emitting diodes arranged in a line by controlling the output intensities of the respective light-emitting diodes while the light-sensitive material is being moved relative to the direction perpendicular to the lengthwise direction of the light-emitting diode array. Specifically, a half-tone image is formed by changing the output light intensity by controlling the currents supplied to the respective light-emitting diodes.
However, the individual light-emitting diodes have different current-intensity characteristics from each other. Therefore, such a current supply circuit as shown in FIG. 1 or FIG. 2 has been used. In the circuit as shown in FIG. 1, a light-emitting diode array 1 is connected between an electric source V.sub.DD and a ground terminal so as to apply a forward bias thereto. Specifically, the cathodes of the respective light-emitting diodes are connected commonly to the ground terminal via resistors and transistors. Control signals S.sub.1 to S.sub.n corresponding to the recording gradation are applied to the respective transistors to thereby control the light-emitting intensities of the respective light-emitting diodes independently of each other. In order to keep the characteristics of the light-emitting diodes uniform, the respective resistors are adjusted to pass an equal current through each light-emitting diode when the control signals S.sub.1 to S.sub.n are at the same level. In the circuit of FIG. 2, a light-emitting diode array 1 is connected between an electric source V.sub.DD and a ground terminal to apply a forward bias. Specifically, transistors are connected between respective ones of the light-emitting diodes and ground through a common resistor 2 so that substantially uniform characteristic are attained for the respective light-emitting diodes.
However, the following problems still exist in the image forming method using such a compensation technique as described above.
In the prior art as shown in FIG. 1, it is very troublesome to adjust the resistors when the light-emitting diodes are large in number. Accordingly, the technique is unsuited for the formation of a high-resolution image. Recent techniques such as laser trimming, used in the field of semiconductor integrated circuits or the like, are very difficult to apply in practice. In the prior art as shown in FIG. 2, it is difficult to form a high-precision image because of the nonlinear current-intensity characteristics of the light-emitting diodes and because of the variations in characteristics of the light-emitting diodes. In addition, the aforementioned conventional apparatus has a problem in that gradation gaps may be recognized by the human eye because the concentration change of the light-sensitive material can not be maintained constant.